1. Field of the Invention
An automated isolate manifold system for purging enclosures used to isolate potentially dangerous electrical switchgear or other equipment from the presence of a hazardous atmospheric environment.
2. Discussion of Related Art
The National Fire Protection Association has established standards for providing purged and pressurized enclosures for containing potentially hazardous electrical equipment. Such enclosures are required in the presence of explosive chemical atmospheres or vapors in chemical plants and combustible materials such as in grain elevators. See for example, publication NFPA 496.
In general, the equipment to be isolated is mounted in a pressurized enclosure. The enclosure is pressurized above the local ambient pressure by means of a remote purge-gas manifolding system. If the enclosure loses its required overpressure such as by the opening of the enclosure or by loss of the inert fluid supply thereof, the equipment controlled by the switchgear of the manifolding system is shut down until the enclosure regains its protective capabilities.
One such system is taught by U.S. Pat. No. 4,802,502, issued Feb. 7, 1989 to Henry L. Williams and assigned to Geolograph Pioneer Inc. Here, a pneumatically operated purge control system provides a non-hazardous environment within an electronics enclosure comprised of a purge air box housing non-electrical components having fluid communication lines between them. An outlet supplies inert gas under pressure from a supply to the enclosure and an inlet receives inert gas under pressure from the enclosure. A pressure regulator, mounted in the box in fluid communication with the supply, provides the air to a manifold distributing air to a flow meter for providing flow of air to the enclosure through the outlet and for determining the rate of flow to the outlet. A signal valve having one inlet communicating with the manifold and another inlet communicating with the inlet; the signal valve having an outlet providing a pneumatic signal when pressure is received at the inlet. A first pneumatic light on the box connected to the outlet of the signal valve indicates the status of the air coming from the enclosure. A time-delay pneumatic relay connected to the outlet of the signal valve provides a delayed signal responsive to the signal received by the signal valve. An explosion-proof pneumatically-operated switch connected to the output of the time delay relay provides electrical power to the enclosure responsive to the delayed signal. A second pneumatic light on the box responsive to the delayed signal indicates the status of the electrical power going into the enclosure. The system includes a two-way valve actuating an alarm system when pressure from the enclosure falls below a predetermined value.
Being substantially mechanical in operation, the '502 system is relatively inflexible in adapting to variable operational episodes. If, for example, the volumetric capacity of the enclosure were changed, such as to accommodate additional electronic modules, the original time delay relay 58 of FIG. 6 of the patent would need to be adjusted or physically replaced by a relay with a different delay. Additionally there appears to be no provision to compensate for enclosure leakage.
U.S. Pat. No. 5,146,105, issued Sep. 8, 1992 to M. Obata et al., teaches an internal pressure explosion proof system by which an electric motor or the like is not rendered operative until after gas in an airtight vessel is replaced with certainty by protective gas. The internal pressure explosion proof system includes a gas flow detector provided in an exhaust pipe for detecting the amount of protective gas exhausted from an airtight vessel in which an electric appliance which may produce a spark or fire, is accommodated and a controller for enabling operation of the electric appliance when a flow amount detected by the gas flow rate detector when protective gas is fed to the airtight vessel reaches a predetermined level and the internal pressure of the airtight vessel exceeds the predetermined level.
U.S. Pat. No. 5,101,710, issued Apr. 7, 1992 to M. K. Baucom and assigned to Bebco Industries Inc. provides a control apparatus or system for purged and pressurized electrical equipment. According to the Abstract, the system provides rapid controlled exchange of pressurizing or purge gas at the electrical enclosure. The control system or apparatus for NFPA type Y and Z electrical equipment enclosure has a source of pressurizing gas , rapid exchange pressure control filter/regulator, rapid exchange pressure gauge and control valve and a manual inlet valve providing rapid exchange of purging gas to the electrical enclosure during start up, an enclosure control valve, enclosure pressure indicator, a venturi providing controlled leakage from the system and a pressure loss control alarm or switch. Another embodiment of the control system or apparatus for NFPA Type X electrical equipment enclosures has a rapid exchange pressure control filter/regulator, rapid exchange pressure gauge, vent control valve and an automatic inlet valve providing for rapid exchange of purging gas to the electrical enclosure during start up and enclosure control valve, enclosure pressure indicator, a venturi providing controlled leakage from the system and a pressure loss alarm or switch. The rapid exchange purging flow is timed by an internal clock or relay to flow for a fixed time dependent only on enclosure size.
A disadvantage of the '710 system is the need for rather complex electrical circuitry. Further, no provision appears to be provided for reprogramming the system so as to adapt a single basic system to the changing needs of a dynamic control system having general applicability. The disclosure teaches that at least seven distinct variants of the control system are required.
There is a need for a flexible control capability for a purge-air control system that is readily programmable to update the system to accommodate changing service environments.